The role of post-sunset vertical drifts at the equator in predicting the onset of VHF scintillations during high and low sunspot activity years

Abstract

Abstract. The day-to-day variability in the occurrence of ionospheric scintillations, which are of serious concern in the trans-ionospheric communications, makes their prediction still a challenging problem. This paper reports on a systematic study in quantitatively identifying the precursors responsible, such as pre-reversal E×B drift velocity, geo-magnetic activity index (Kp) and the Equatorial Ionization Anomaly (EIA) gradient, for the onset of VHF scintillations over a low-latitude station, Waltair (20° N dip), during high (2001) and low (2004) sunspot activity years. The percentage of occurrences of VHF scintillations over Waltair show a good correlation with the monthly mean post-sunset vertical drift velocities at the equator, during both the high and low sunspot activity years. During the days on which intense (>10 dB) scintillations occur, the ionization anomaly gradient (dN/dL), measured from ionosonde data of an equatorial (Trivandrum, 0.9° N dip) and an off-equatorial station (Waltair, 20° N dip) shows an enhancement in the gradient prior to the onset of the scintillations. However, this enhancement is not seen on days when the scintillations are weak (<10 dB) or absent. The day-to-day post sunset enhancement in the E×B drift is found to decrease with increasing Kp-index and this decrease is more prominent in the equinoxes, less in winter and insignificant in the summer months. On a day-to-day basis, it is found that the value of the upward drift velocity at the equator should be ≥30 m/s for the onset of strong scintillations over Waltair for magnetically quiet days with average Kp≤2 (6 h prior to the local sunset) during the high sunspot year, 2001. This threshold value of the upward drift reduces to 20 m/s with the decrease in the sunspot activity during 2004. Further, these conditions for the onset of intense scintillations is well defined in equinoxes, less in winter and least in the summer solstices.

The seasonal and solar cycle variations of electron density gradient scale length, vertical drift and layer height during magnetically quiet days: Implications for Spread F over Trivandrum, India

“…They had used a constant value of L for all the growth rate estimates. The seasonal and solar cycle variability of ESF obtained in the present study agrees with that obtained in previous studies (Subbarao and Krishna Murthy, 1994;Tulasi Ram et al, 2006). The inclusion of L variability also in the growth rate estimates may be able to give better indication of ESF occurrence probability especially while considering on a dayto-day basis.…”

“…d(h F) is the difference between the maximum layer height in the post sunset period and the layer height at 1800 hr. Here the vertical drift due to recombination effects is neglected as it is not significant in relation to the large vertical drift due to pre-reversal enhancement of the zonal electric field (Tulasi Ram et al, 2006). The L and maximum d(h F)/dt values are estimated for available data on each day, corresponding to the three seasons of solar minimum (1995)(1996) and solar maximum (2002)(2003) years.…”

Characteristics of equatorial ionization anomaly (EIA) in relation to transionospheric satellite links around the northern crest in the Indian longitude sector

“…Several workers have studied the plausible causes and effects (Ray et al, 2006; , 2001, 2004Rama Rao et al, 2006;Ram et al, 2006;Nava et al, 2007) of such steep TEC gradients and have strived to develop a predictive mechanism for occurrence for post-sunset scintillations. These exercises have mostly been restricted to the region between the magnetic equator and the northern and southern crests of EIA.…”

“…The impact of a developed equatorial ionization anomaly (EIA) in the afternoon hours on the subsequent development of equatorial ionization density irregularities during postsunset hours was suggested by Raghavarao et al (1988) and validated by several workers (Valladares et al, 2001(Valladares et al, , 2004Ray et al, 2006;Rama Rao et al, 2006;Ram et al, 2006;Nava et al, 2007). However the majority of the validation exercise was performed at locations existing between the magnetic equator and the northern and southern crests of the EIA.…”

The combined effects of electrojet strength and the geomagnetic activity (<I>K<sub>p</sub></I>-index) on the post sunset height rise of the F-layer and its role in the generation of ESF during high and low solar activity periods

“…Several earlier studies have also shown that a rise in the height of the nighttime F-layer is the most important parameter controlling the generation of spread-F (Farley et al, 1970;Ossakow et al, 1979;Rastogi, 1980;Sastri, 1984;Kelley and Maruyama, 1992;Devasia et al, 2002;Jyoti et al, 2004). Further, Fejer et al (1999), Anderson et al (2004) and Tulasi Ram et al (2006) have shown that, when the post-sunset vertical drifts at the equator are large enough, the necessary condition to trigger the ESF always appears to be present.…”